Device for neutralizing temperature dependent gain variations in an optically pumped laser rod

ABSTRACT

The mirror system of a resonator for a solid state laser includes a totally reflecting prism the principal section of which is a right-angled isosceles triangle and which has its hypotenuse side facing one end of the laser rod so that an incident beam from the laser is reflected back into the laser rod in parallel with the direction of incidence. A temperature sensitive device senses the temperature in the neighborhood of the laser rod and actuates the prism so that its 90* corner is displaced in relation to the center axis of the laser rod in response to temperature changes to cause a reflected beam to travel through a part of the laser rod having an excitation level appropriate for the desired optical gain.

United States Patent Bergqvist [.4 Sept. 5, 1972 [54] DEVICE FORNEUTRALIZING 3,476,463 11/1969 Kreuzer ..350/ 189 TEMPERATURE DEPENDENTGAIN 3,484,715 12/ 1969 Rempel ..33l/94.5 IA S IN AN OPTIC ALLY3,478,608 1 l/ 1969 Met ..33 l/94.5 X

Primary ExaminerRonald L. Wibert Assistant ExaminerEdward S. BauerAtt0rney-Hane, Baxley & Spiecens 7] ABSTRACT The mirror system of aresonator for a solid state laser includes a totally reflecting prismthe principal section of which is a right-angled isosceles triangle andwhich has its hypotenuse side facing one end of the laser rod so that anincident beam from the laser is reflected back into the laser rod inparallel with the direction of incidence. A temperature sensitive devicesenses the temperature in the neighborhood of the laser rod and actuatesthe prism so that its 90 comer is displaced in relation to the centeraxis of the laser rod in response to temperature changes to cause areflected beam to travel through a part of the laser rod having anexcitation level appropriate for the desired optical gain.

4 Claims, 4 Drawm' g Figures PUMPED LASER ROD [72] Inventor: Erik ArneBergqvist, Karlskoga,

Sweden [73] Assignee: Aktiebolaget Bofors, Bofors,

Sweden [22] Filed: April 23, 1970 21 Appl. No.: 31,261

[30] Foreign Application Priority Data May 6, 1969 Sweden ..6422/69 [52]U.S. Cl ..331/94.5 [51] Int. Cl ..H01s 3/00 [58] Field of Search..33l/94.5

[56] References Cited UNITED STATES PATENTS 3,055,257 9/1962 Boyd et al..33l/94.5

PKTENTEDSEP 5 I372 I FIG. '2

FIG. l

FIG. 3

DEVICE FOR NEUTRALIZING TEMPERATURE DEPENDENT GAIN VARIATIONS IN ANOPTICALLY PUMPED LASER ROD BACKGROUND OF THE INVENTION the temperaturedependent gain variation in the laser 1 material.

A laser crystal is strongly influenced by temperature and thisparticularly applies to a ruby laser. This depends on the atomicproperties of the laser crystal. Thus, the duration of the metastablestate in the laser crystal decreases with increasing temperature at thesame time as the amplification of the stimulated emission (the laseraction) decreases. For this reason a high excitation energy (pumpingenergy) is required at high temperatures. If the laser-is to operatewithin a large temperature range, e.g. from 50 to +60 C., and theexcitation energy is chosen sufficiently large to make the laser worksatisfactorily at the upper limit of the temperature range, thisexcitation energy will be high at low temperatures. This causesdifficulties particularly in Q-pulsed lasers, since, due to the highgain at low temperatures, double pulses are produced if the appliedpumping energy is high. In order that single pulses shall be obtainedwhich is particularly desirable if the laser is used for distancemeasuring purposes, the

pumping energy at low temperatures can be decreased, but the singlepulses then obtained will have a low energy. In order that single pulseshaving a high energy shall be obtained even at low temperatures theoptical gain of the laser must be decreased in relation to the gain athigher temperatures.

An object of the present invention is to provide a SUMMARY OF THEINVENTION The invention utilizes the fact that the excitation levelproduced by the pumping with an optical pumping system is different indifferent partsof the laser body. In most lasers with optical pumpingsystems the distribution of excited atoms in the laser rod is such thatthe center of the rod is excited to the highest degree, while theexcitation level adjacent to the surface of the rod is barely sufficientto cause laser action. The distribution of the excitation in the laserrod is determined by the geometry of the pumping system and of thedegree of doping of the laser material. If for instance, the pumpingsystem is provided with an elliptii cal reflector and the laser rod isarranged with its center axis coinciding with one focal line of thereflector and a rod-shaped pumping lamp is arranged along the otherfocal line of the reflector, the excitation level will decrease nearlyuniformly from a maximum at the center of the rod in all directionsradially towards the surface of the rod.

The excitation level determines the optical gain and therefore this willvary within the laser body in the same sense as the excitation level.According to the invention this fact is utilized by actuatingmechanically the mirror system of the resonator so. that beams radiatedfrom the laser body after reflection in the mirror system returns tosuch parts of the laser body which have an excitation level suited tothe desired gain. In a preferred embodiment of the invention the mirrorsystem is actuated automatically in response to the temperature of theenvironment of the laser body so that the gain is decreased withdecreasing temperature. 1

In Q-pulsed lasers the totally reflecting mirror of the resonatorusually consists of a fixed or rotatable prism the principal section ofwhich is a right-angled isosceles triangle. Such a prism is also knownas the Porroprism. By means of such a prism a gain control according tothe invention can be easily effected by making the prism movable so thatits comer can be displaced at right angles to the longitudinal directionof thelaser rod whereby a variation of the optical gain is obtained aswill be described more in particular in the following. The displacementof the prism can be effected by means of actuating members which arecontrolled by temperature sensitive devices so that the gain is variedin accordance with the temperature.

DRAWINGS AND DETAILED DESCRIPTION The invention will be described morein particular with reference to the accompanying drawing.

FIGS. 1 and 2 schematically illustrate the principle of the invention.

FIGS. 3 and 4 show schematically two difierent devices forautomaticallycontrolling the gain of the laser in response to the temperature.

FIGS. 1 and 2 show a laser rod 1, which may consist of e.g. a rod-shapedruby. At one end of the laser rod 1 is located a totally reflectingprism 2 of the kind described above. The other elements belonging to acomplete laser system such as pumping system and the semi-transparentmirror arranged at the other end of the laser rod are assumed to be ofwell-known design, and therefore they have not been shown in thedrawing.

Above the laser rod 1 in FIGS. 1 and 2 there is a diagram diagramshowing how the excitation level and hence the optical gain varies alongthe cross section of the laser rod. As will be seen from this diagramthe excitation has a maximum along the center axis of the laser rod anddecreases practically symmetrically with increasing distance from thecenter line.

In FIG. 1 the prism 2 is shown having its 90 corner situated at theextension of the center axis of the laser rod 1, Le. the 90 comerregisters with the excitation maximum in the laser rod. When the prism 2is in this position in relation to the laser rod 1 a beam emitted fromthe laser rod will be reflected by the prism 2 into the laser rod alonga path in which the excitation level is about the same as in the path oftravel of the outgoing beam through the laser rod. Hereby a maximumoptical gain is obtained in the laser rod. If the laser operates at ahigh temperature the prism 2 should therefore assume the position shownin FIG. 1 in relation to the laser rod 1 in order that the laser shalloperate satisfactorily with the lowest possible pumping energy.

such case the generated single pulses will have too low an energy.

According to the invention the optical gain at low temperatures isdecreased by displacing the prism 2 so that its 90 corner is no longerregistering with the excitation center. This is illustrated in FIG. 2 inwhich prism 2 is shown in such position that its 90 comer is situated ata perpendicular distance d from the excitation center in the laser rod1.

When total reflection occurs in a prism of the shown kind the reflectedbeam will be parallel to the incident beam. The distance between theincident and the reflected beam is determined by the distance betweenthe incident beam and the 90 corner of the prism and is equal to twicethe said distance. The displacement of the prism in relation to theexcitation center of the laser rod as shown in FIG. 2 consequentlycauses a beam which passed through the excitation center before enteringthe prism to return after reflection in the prism through a path in thelaser rod 1 having a lower excitation. The amplification of the lightupon the double passage through the laser rod (this amplification beingproportional to the product of the amplifications upon the two singlepassages) in this case is lower than in the case when the 90 corner ofthe prism registers with the excitation center of the laser rod. Thisdecrease of the amplification makes it possible to generate singlepulses having a high energy even at low temperatures.

In order to fulfil the requirements of a low pumping energy at hightemperatures and a high energy of single pulses at low temperatures theposition of the prism 2 must vary with temperature. To this end theprism 2 may be mounted in a holder which can be displaced e.g. manuallyby means of a screw device. However, the required change of position ofthe prism 2 in response to the temperature may also be effectedautomatically by means of suitable temperature sensitive means. FIGS. 3and 4 show examples of such means.

In the device shown in FIG. 3 the prism 2 is mounted in a holder 3 whichis suspended at joints 6 and 7 in two bimetallic springs 4 and which areattached to a fixed base 8. The arrangement is such that the prism 2assumes the position shown in FIG. 1 in relation to the laser rod at themaximum temperature at which the laser is intended to operate. When thetemperature decreases the bimetallic springs 4 and 5 are bent so thatthe prism 2 is carried away from this position.

In the arrangement shown in FIG. 4 the prism 2 is likewise mounted in aholder 3 which can slide on a fixed support 15. An elongated container 9is mounted adjacent to the laser rod 1. This container 9 is filled witha liquid 10 which preferably is mercury. The container 9 communicatesthrough a tube 11 with a cylinder 12 in which a piston 13 is slidablymounted, the piston rod being connected to the prism holder 3. Apressure spring 14 is arranged between one end of the prism holder 3 anda fixed support 16. When the temperature is changed the volume of liquid10 is also changed whereby the prism holder 3 will be displaced underthe action of piston 13 and spring 14. In emicontinuous operation when etemperature 0 the laser rod may appreciably exceed the ambienttemperature, the container 9 should preferably be located near to thelaser rod 1, whereby the temperature of the laser rod can be bettersensed than by the device shown in FIG. 3.

In Q-pulsed lasers the totally reflecting prism 2 is often used for thepulsing in which case the prism is caused to rotate about an axis atright angles to the vertex of the prism. In this case the drive meansfor causing the prism to rotate may be mounted together with the prismon a suitable holder which is displaced in response to the temperaturein the manner described above.

The means for displacing the prism 'in response to temperature'changesmay of course be designed in many other ways than those illustrated inFIGS. 3 and 4.

What is claimed is:

l. A device for controlling the amplification in a laser systemincluding an elongated solid laser body of the kind in which theexcitation level produced by optical pumping action varies along thecross-section of the laserbody, said device comprising: a prism adjacentone end of the laser body for reflecting radiation emitted from thelaser body back into the laser body, said prism having a principalsection which is a right angled isosceles triangle; and actuating meansfor transversely moving said prism so that its 90 corner is displacedalong a line which is at right angles to the center line of the laserbody and which is at right angles to said corner, said actuating meanscomprising a temperature sensitive means sensitive to the temperature inthe environment of the laser body for moving said prism so that the 90?corner thereof moves along said line in response to this temperature toneutralize the temperature dependent gain variations.

2. A device as claimed in claim 1 wherein the actuating means includingthe temperature sensitive means is so arranged that at a relatively hightemperature in the environment of the laser body the prism is kept insuch position that its 90 corner is opposite a range of maximumexcitation in the laser body and upon increasing temperature istransversely moved from this position.

3. A device as claimed in claim 1 wherein said temperature sensitivemeans comprises a bimetallic element connected to said prism in such away that when said bimetallicelement changes shape in response tochanges in temperature in the environment of the laser body the 90corner of said prism moves transversely along said line.

4. A device as claimed in claim 1 wherein said temperature sensitivemeans comprises a container for a liquid whose volume is a function oftemperature, a piston device connected to said container and moved bythe liquid when the volume of the liquid is changed with temperaturechanges, and means connecting said piston to said prism for moving the90 corner thereof along said line as said piston is moved by the liquidin said container.

II l

1. A device for controlling the amplification in a laser systemincluding an elongated solid laser body of the kind in which theexcitation level produced by optical pumping action varies along thecross-section of the laser body, said device comprising: a prismadjacent one end of the laser body for reflecting radiation emitted fromthe laser body back into the laser body, said prism having a principalsection which is a right-angled isosceles triangle; and actuating meansfor transversely moving said prism so that its 90* corner is displacedalong a line which is at right angles to the center line of the laserbody and which is at right angles to said corner, said actuating meanscomprising a temperature sensitive means sensitive to the temperature inthe environment of the laser body for moving said prism so that the 90*corner thereof moves along said line in response to this temperature toneutralize the temperature dependent gain variations.
 2. A device asclaimed in claim 1 wherein the actuating means including the temperaturesensitive means is so arranged that at a relatively high temperature inthe environment of the laser body the prism is kept in such positionthat its 90* corner is opposite a range of maximum excitation in thelaser body and upon increasing temperature is transversely moved fromthis position.
 3. A device as claimed in claim 1 wherein saidtemperature sensitive means comprises a bimetallic element connected tosaid prism in such a way that when said bimetallic element changes shapein response to changes in temperature in the environment of the laserbody the 90* corner of said prism moves transversely along said line. 4.A device as claimed in claim 1 wherein said temperature sensitive meanscomprises a container for a liquid whose volume is a function oftemperature, a piston device connected to said container and moved bythe liquid when the volume of the liquid is changed with temperaturechanges, and means connecting said piston to said prism for moving the90* corner thereof along said line as said piston is moved by the liquidin said container.